I can see it now. After half an hour of build-up, with the Royal family and assorted dignitaries arriving at St Paul’s this morning for the Service of Thanksgiving, the choir will rise to begin Vaughan Williams’s Te Deum.

And at that precise moment the BBC will cut across to more important matters: Fearne Cotton interviewing the members of boy band One Direction on how they’ve been marking the Jubilee. If the BBC’s coverage of Sunday’s flotilla is anything to go by, the last thing we will get this morning is coverage of the service itself. Or perhaps, if the producer does consent to allow some pictures to come from inside the cathedral, there’ll be a commentary from a daytime TV presenter explaining that a cathedral is a big church with lots of people and although the service might go on a little while, don’t worry, there’s some music with nice tunes.

Like many viewers, I watched the BBC on Sunday with incredulity and mounting anger. It has become a truism that our national culture has been infantilised and made stupid. But if ever anything could be relied on to provide a temporary halt in that slide it would, surely, be the BBC’s coverage of the Diamond Jubilee. Much to the irritation of other channels, we turn to the national broadcaster at times of national togetherness. The BBC just gets it right.

Not any more. Sunday’s broadcast was not merely inane, it was insulting. The instruction had clearly gone out from on high that the audience would comprise imbeciles with a mental age of three and a 20-second attention span. And that any celebrity sighting, no matter how minor, would trump anything happening on the river.

So the flotilla – an event so awe-inspiring that it drew well over a million people, on a cold wet day, to stand 10-deep on the banks of the Thames to try to catch a sight – was treated merely as background for the witterings of the BBC’s most lightweight presenters and the D-list celebrities they had lined up to lurk anywhere but on the river. How else to explain the decision to cut away from the flotilla just as the Spirit of Chartwell, with the Royal family aboard, sailed through Tower Bridge with its gates up – one of the highlights of the afternoon – and switch to an interview by Fearne Cotton so cringe-makingly inept that it should be shown to all wannabe presenters as an example of how good looks are not enough. Miss Cotton was with some Second World War veterans, but appeared to have no idea what that war was or why she was interviewing the men. “This is obviously a huge ship, this is a weighty ship. How have you enjoyed your day cheering the Queen?”

At various points during the day we were removed from the flotilla to hear from Sandi Toksvig, Griff Rhys Jones and Omid Djalili. Tess Daly, the Strictly Come Dancing presenter, was in Battersea Park with a bunch of cross-dressers (no, I’ve no idea why either). Anneka Rice was standing on the Millennium Bridge with some amateur artists. And there was an interview with a clearly startled new mother at St Thomas’ Hospital (it’s on the river, geddit?). The intellectual high-water mark was the former political correspondent John Sergeant talking to the actor Richard E Grant, who then read some Wordsworth.

Anything, you see, except coverage of the flotilla itself. And absolutely anything except informing viewers about the history, significance and provenance of the boats and those people actually taking part in the flotilla. I challenge the BBC to find a single viewer – seriously, just one – who preferred interviews with celebrities to coverage of the flotilla. Instead of heavyweights such as David Dimbleby and Andrew Marr, who have years of experience, a deep fount of knowledge and who are peerless at putting it across to viewers, we had Matt Baker, a presenter of The One Show, with newsreader Sophie Raworth as the fulcrum of the coverage. The One Show is decent, lightweight magazine fare. Lightweight is being charitable. They might as well have dragged a couple of children off the streets, for all the information they imparted. It seemed almost as if their ignorance was deliberate, as they wondered aloud about key issues such as when the royal party would get a lavatory break.

The deep-seated cluelessness began when a commentator referred to the Queen as “Her Royal Highness” rather than Her Majesty. In a broadcast about the Queen, the presenter could not even be bothered to find out how to refer to the monarch. But in the great swill of drivel spewed up by the BBC on Sunday, that was the least of it. My own favourite was the revelation provided by the main commentator, Paul Dickenson, that “Tower Bridge is about as iconic as it gets”.

Tom Fleming, the man who graced so much of the BBC’s fine coverage of state occasions in days long past, must be turning in his grave. The BBC’s role as the pre-eminent national broadcaster is, on this evidence, surely over.